Variable communication system

ABSTRACT

A communication amount monitoring circuit ( 124 ) monitors a transmission buffer ( 123 ) to which communication data ( 122 ) is input, and discriminates the magnitude of the amount of communication data. If the amount of information per unit time is relatively large, information is read out from the transmission buffer in accordance with a chip clock. This information is modulated as communication data ( 137 ) by a modulator ( 139 ) and transmitted. If the amount of information is relatively small, information is read out from the transmission buffer ( 123 ) in accordance with a clock obtained by frequency-dividing the chip clock. This information is exclusive-ORed with a spreading code generated by using the chip clock and is then modulated by a modulator ( 139 ). The resultant data is transmitted as communication data ( 137 ). Since there is no change in the frequency of the communication data ( 137 ), there is no need to provide any resynchronization on the reception device side at the time of switching between the large and small amounts of information. There is therefore no need to provide any buffer area for resynchronization.

BACKGROUND OF THE INVENTION

The present invention relates to a digital communication system and,more particularly, to a variable communication system which switchescommunication schemes in accordance with the communication amount.

In digital communication systems, there have been various proposals toefficiently transmit data. For example, in the digital communicationsystem disclosed in Japanese Patent Laid-Open No. 7-123039, the trafficof communication is detected from an m-sequence signal input to a rateconversion multiplexer (not shown), and the rate conversion multiplexermultiplexes data at a rate corresponding to the traffic. A variable ratemodulator (not shown) then modulates the multiplexed signal andtransmits it to a transmission path at a transmission rate correspondingto the magnitude of the traffic. That is, the modulation clock rate istime-modulated. On the receiving side, a variable rate demodulator (notshown) performs demodulation, and a rate conversion demultiplexer (notshown) demultiplexes and rate-converts the demodulated signal toreproduce the data. Claim 2 in Japanese Patent Laid-Open No. 2000-316035proposes a technique of efficiently assigning bands by dynamicallychanging the transmission rate in accordance with the transmissionamount as in the above prior art.

In this manner, in the field of digital communication systems, when thecommunication amount is small, the transmission rate of information tobe transmitted is set low to set a line with a low modulation clockrate, whereas as the communication amount increases, a line with a highmodulation clock rate and high transmission rate is set. For example, arouter which has selected a general voice line connects to apredetermined content server on the Internet at a given time point totransfer large-volume data. Likewise, when a cell phone switches to thevideophone mode during voice communication, the communication amountincreases at this time point. In a case opposite to the above case, thecommunication amount decreases at a predetermined time point.

Properly switching the transmission rates of information in accordancewith the communication amount in this manner makes it possible toperform efficient communication. In general, however, when themodulation scheme to be used remains the same, if the transmission rateis low and the modulation clock rate is low, a line tends to relativelydeteriorate because it is susceptible to the influences of phase noisein a local oscillator in an up converter which performs frequencyconversion to obtain carrier waves or a down converter in a receptionunit. This makes it impossible to obtain a stable transmission path. Ingeneral, therefore, it is preferable to use a line with a hightransmission rate and high modulation clock rate.

In contrast, it is conceivable to use a technique of performingmultilevel modulation in the same band by different modulation schemeswhile the modulation clock rate is kept constant. 16-QAM (QuadratureAmplitude Modulation) which can obtain 16 signal states is capable ofobtaining a quadruple transmission rate at the same modulation clockrate as compared with BPSK (Bi-Phase Shift Keying) which is a modulationscheme of making bit information of a baseband correspond to two phases.When, however, multilevel modulation is to be performed, in order tokeep transmission path quality (i.e., a bit error rate) equivalent to amodulation scheme with a low transmission rate, it is necessary toincrease the output level by an amount equal to or more than a bandimprovement in terms of transmission rate. In the above case, although16-QAM realizes a band improvement of 6 dB in terms of transmission rateas compared with BPSK, the transmission carrier power must be increasedby 10 dB or more to obtain the same bit error rate at 10E-6 as that inBPSK.

In addition, when the transmission rates are switched in accordance withthe communication amount, demodulation synchronization must beestablished with respect to the transmission rate after switching at thetime of this switching operation. Demodulated data is temporarilyinterrupted until demodulation synchronization is established. Compare acase wherein the transmission rate after switching is high with a casewherein the transmission rare after switching is low. In the lattercase, for example, in a carrier wave reproducing circuit, the frequencyerror of a reception carrier wave with respect to the modulation clockrate becomes relatively large. For this reason, in a PLL circuit forcarrier wave reproduction, it takes a longer time to establishdemodulation synchronization than in the case wherein the transmissionrate after switching is high. Omission of communication data due to theinterruption of demodulated data leads to a crucial result on thereproduction of information. In order to prevent omission of informationdue to the interruption of demodulated data, a buffer memory has beenconventionally used. There has been a communication sequence in whichwhen transmission or reception is to be performed, communication data istemporarily stored in the buffer memory, and transmission is stopped ata predetermined timing which poses no problem in the reproduction ofinformation. In this sequence, when the receiving side detects omissionof communication data, a re-transmission request is generated to readout the corresponding portion from the buffer memory on the transmittingside and re-transmit it.

In order to prevent omission of communication data and guarantee theperfection of reproduction of information by using such a technique, aconsiderably large buffer memory must be prepared. If information ratesare set in two levels, i.e., a low rate and a high rate, as in the abovecase, in particular, the interruption of communication data becomeslarge in the case wherein the information rate is low. In this case,therefore, a buffer memory large enough to cover this must be prepared.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a variablecommunication system which reduces the capacity of a buffer memoryascribed to switching of transmission rates while switchingcommunication schemes in accordance with the magnitude of thecommunication amount per unit time.

In order to achieve the above object, according to the presentinvention, there is provided a variable communication systemcharacterized by comprising (I) a transmission device includinginformation amount magnitude discrimination means for discriminatingwhether an amount of information to be transmitted per unit time isrelatively large or small, first communication data sending means for,when the information amount magnitude discrimination means discriminatesthat the amount of information to be transmitted is relatively large,digitally modulating first information as the information intoinformation in a signal form having a predetermined bandwidth with apredetermined center frequency, and sending out the information ascommunication data, and second communication data sending means for,when the information amount magnitude discrimination means discriminatesthat the amount of information to be transmitted is relatively small,digitally modulating second information as the information uponperforming spread spectrum to obtain the same bandwidth as thepredetermined bandwidth with the center frequency, and sending out theinformation as the communication data, and (II) a reception deviceincluding demodulation means for demodulating the communication datasent from the transmission device, de-spreading appropriatenessdiscrimination means for checking whether or not a signal afterdemodulation can be normally de-spread, first information reproductionmeans for, when the de-spreading appropriateness discrimination meansdiscriminates that de-spreading cannot be normally performed,reproducing the first information from the signal after demodulation bythe demodulation means, de-spreading means for, when the de-spreadingappropriateness discrimination means discriminates that de-spreading canbe normally performed, de-spreading the signal after demodulation by thedemodulation means, and second information reproduction means forreproducing the second information from the signal after de-spreading bythe de-spreading means.

That is, the transmission device includes the information amountmagnitude discrimination means and discriminates whether the amount ofinformation to be transmitted per unit time is relatively large orsmall. If it is discriminated from this discrimination result that theinformation is relatively large, the first communication data sendingmeans digitally modulates the first information into information in asignal form having a predetermined bandwidth with a predetermined centerfrequency, and sends out the resultant data as communication data. Inthis regard, there is no specific difference from the prior art. If theinformation amount magnitude discrimination means discriminates that theinformation to be transmitted is relatively small, the secondinformation as this information is subjected to spread spectrum anddigital modulation to be sent out as communication data. In this case,the necessary bandwidth is spread to the bandwidth associated with thefirst signal by spread spectrum. The center frequency of the secondinformation is also made equal to that of the first information, and thesecond information is sent out as communication data. This makes itunnecessary for the demodulation means of the reception device to switchfrequencies for the first information and the second information. Thetime required for this operation is also made unnecessary. In thereception device, the de-spreading appropriateness discrimination meanschecks whether or not a signal after demodulation can be normallyde-spread. If it is determined that de-spreading can be done, thissignal is regarded as the second information and de-spread to bereproduced. If it is determined that de-spreading cannot be done, thissignal is regarded as the first information and reproduced withoutperforming any spread spectrum.

In the above variable communication system, the transmission device mayfurther comprise a transmission buffer which sequentially receivesinformation to be transmitted and outputs the information in synchronismwith a predetermined read clock, and the information amount magnitudediscrimination means may discriminate from an amount of information leftin the transmission buffer whether the amount of information per unittime is relatively large or small. In addition, the first and secondcommunication data sending means of the transmission device may includechip clock generating means for outputting a chip clock having apredetermined frequency, transmitting-side frequency dividing means forfrequency-dividing the chip clock output from the chip clock generatingmeans at a predetermined frequency division ratio, read clock selectionmeans for, when the information amount magnitude discrimination meansdiscriminates that information to be transmitted is relatively large,setting the chip clock as the read clock, and when the informationamount magnitude discrimination means discriminates that information tobe transmitted is relatively small, setting, as the read clock, a clockobtained by frequency-dividing the chip clock by the transmitting-sidefrequency dividing means, spreading code generating means for receivingthe chip clock and generating a spreading code, transmitting-side switchmeans which receives an output from the spreading code generating meansand is turned on only when the information amount magnitudediscrimination means discriminates that information to be transmitted isrelatively small, transmitting-side exclusive addition means forcalculating exclusive-OR between information output from thetransmission buffer in synchronism with the read clock and an outputfrom the transmitting-side switch means, and modulation means fordigitally modulating an output from the transmitting-side exclusiveaddition means and transmitting the output as the communication data.Furthermore, the de-spreading means and the first and second informationreproduction means of the reception device may include reception clockgenerating means for outputting a reception clock identical to the chipclock, de-spreading code generating means for generating a de-spreadingcode on the basis of the reception clock output from the reception clockgenerating means, receiving-side switch means which receives an outputfrom the de-spreading code generating means and is turned on only whenthe de-spreading appropriateness discrimination means discriminates thatde-spreading can be performed, receiving-side exclusive addition meansfor calculating exclusive-OR between an output from the receiving-sideswitch means and a signal after demodulation by the demodulation means,receiving-side frequency dividing means for frequency-dividing thereception clock at the predetermined frequency division ratio, writeclock selection means for, when the de-spreading appropriatenessdiscrimination means discriminates that de-spreading cannot beperformed, selecting the reception clock, and when de-spreadingappropriateness discrimination means discriminates that de-spreading canbe performed, selecting and outputting a clock obtained byfrequency-dividing the reception clock by using the receiving-sidefrequency dividing means, and a reception buffer in which an output fromthe receiving-side exclusive addition means is written as an input inaccordance with the write clock selected by the write clock selectionmeans, and data stored in the reception buffer is set as the informationto be transmitted.

That is, the transmission device includes the transmission buffer, andthe information amount magnitude discrimination means discriminates fromthe amount of information left in the transmission buffer whether theamount of information input to the device per unit time is relativelylarge or small. In accordance with this discrimination result, theperiod of a read clock for reading out data from the transmission buffercan be changed in two levels. More specifically, if the informationamount magnitude discrimination means discriminates that the informationto be transmitted is relatively large, a chip clock is used as a readclock. If the information amount magnitude discrimination meansdiscriminates that the amount of information is relatively small, a slowclock is obtained by frequency-dividing this chip clock using thetransmitting-side frequency dividing means. The data read out from thetransmission buffer in this manner becomes one input to thetransmitting-side exclusive addition means. As the other input, thespreading code generated on the basis of the chip clock is input throughthe transmitting-side switch means. In this case, the transmitting-sideswitch means is a switch which is turned on only when the informationamount magnitude discrimination means discriminates that the informationto be transmitted is relatively small. With this arrangement, if theinformation amount magnitude discrimination means determines that theinformation to be transmitted is relatively large, the modulation meansoutputs a signal, as communication data, which has a bandwidthcorresponding to the information amount. If the information amountmagnitude discrimination means determines that the amount of informationis relatively small, spread spectrum is performed to spread thebandwidth, and the resultant data is output as communication data, whichhas the same center frequency as that in the case wherein it isdetermined that the amount of information is relatively large. For thisreason, the transmission rate remains the same regardless of the form ofcommunication data received, the demodulation means on the receptiondevice side need not perform demodulation synchronization again. If itis checked on the basis of demodulated data after demodulation by thedemodulation means whether spread spectrum can be normally performed,and it is determined that spread spectrum can be performed, the sentsignal is regarded as a signal sent by spread spectrum. This signal isthen de-spread and written in the reception buffer. Otherwise, thedemodulated data after demodulation is written in this reception buffer.In this case, the write clock selection means selects a write clock inaccordance with the rate read out from the transmission buffer on thetransmission device side. In this manner, the information afterdemodulation which is to be transmitted is obtained from the receptionbuffer.

Note that the first and second communication data sending means of thetransmission device may include chip clock generating means foroutputting a chip clock having a predetermined frequency,transmitting-side frequency dividing means for frequency-dividing thechip clock output from the chip clock generating means at apredetermined frequency division ratio, read clock selection means for,when the information amount magnitude discrimination means discriminatesthat information to be transmitted is relatively large, setting the chipclock as the read clock, and when the information amount magnitudediscrimination means discriminates that information to be transmitted isrelatively small, setting, as the read clock, a clock obtained byfrequency-dividing the chip clock by the transmitting-side frequencydividing means, modulation means for digitally modulating informationoutput from the transmission buffer in synchronism with a read clock,spreading code generating means for receiving the chip clock andgenerating a spreading code, transmitting-side switch means whichreceives an output from the spreading code generating means and isturned on only when the information amount magnitude discriminationmeans discriminates that information to be transmitted is relativelysmall, transmitting-side exclusive addition means for calculatingexclusive-OR between an output from the modulation means and an outputfrom the transmitting-side switch means and transmitting the data as thecommunication data.

In addition, the reception clock generating means of the receptiondevice may comprise reception clock reproduction means for reproducing areception clock from communication data input to the demodulation means.

Furthermore, the information amount magnitude discrimination means ofthe transmission device may set a predetermined difference between athreshold by which it is discriminated that an information amount isrelatively large and a threshold by which it is discriminated that aninformation amount is relatively small. More specifically, theinformation amount magnitude discrimination means may set a threshold bywhich it is discriminated that an information amount is relatively largeto a value larger than a threshold by which it is discriminated that aninformation amount is relatively small.

In addition, the information amount magnitude discrimination means ofthe transmission device may discriminate, depending on whether or not apath through which information to be transmitted is acquired is apre-specified path, whether the amount of information to be transmittedper unit time is relatively large or small. Alternatively, theinformation amount magnitude discrimination means may discriminate,depending on whether or not a device which processes information to betransmitted is set in a pre-specified mode, whether the amount ofinformation to be transmitted per unit time is relatively large orsmall.

Furthermore, the transmission device and the reception device maycomprise a radio device.

Moreover, the transmission device may output transmission power inproportion to a transmission rate. When the transmission rate is low,since the transmission power is decreased, the transmission powerdensity decreases by an amount corresponding to an increase in occupiedbandwidth due to spread spectrum.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system configuration diagram showing the overall arrangementof a variable communication system according to the first embodiment ofthe present invention;

FIG. 2 is a block diagram showing the arrangement of the transmissionunit of the first communication device according to this embodiment;

FIG. 3 is a block diagram showing the arrangement of the reception unitof the second communication device according to this embodiment;

FIGS. 4A and 4B are graphs showing comparison between spectra in a casewherein switching is performed in accordance with the communicationamount in this embodiment;

FIGS. 5A and 5B are graphs showing comparison between spectra in a casewherein switching is performed in accordance with the communicationamount in the prior art;

FIG. 6 is a flowchart showing how switching control is performed whencommunication methods are switched in accordance with the amount ofcommunication data in a cell phone according to the third embodiment ofthe present invention; and

FIG. 7 is a block diagram showing the arrangement of the transmissionunit of the first communication device in the fourth embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments of the present invention will be described in detailbelow with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows the overall arrangement of a variable communication systemaccording to the first embodiment of the present invention. Thisvariable communication system is formed as a communication system inwhich two LANs (Local Area Networks) including a first LAN 101 and asecond LAN 102 are connected to each other. A host computer (HOST) 103and first router (Router) 104 are connected to the first LAN 101. Afirst communication device 106 for performing communication with a radiocommunication channel 105 is connected to the first router 104. Aterminal 107 controlled by a host computer (not shown) and a secondrouter 108 are connected to the second LAN 102. A second communicationdevice 109 for performing communication with the radio communicationchannel 105 is connected to the second router 108.

In this variable communication system, the first and secondcommunication devices 106 and 109 are designed to monitor communicationamounts when they perform communication using the radio communicationchannel 105. Each communication device performs communication uponswitching information rates depending on whether the communicationamount is relatively large or small. Note, however, that the transferrate of communication data communicated through the radio communicationchannel 105 is always kept constant, and when the communication amountis relatively large, a general PSK (Phase Shift Keying) modulationscheme is used to transmit information. When the communication amount isrelatively small, spread-spectrum communication is performed. Since thetransfer rate of communication data remains unchanged, even if thecommunication amount varies, transmission rate resynchronization is notrequired at the time of switching transmission rates. This eliminatesany communication interruption time.

FIG. 2 shows the arrangement of the transmission unit of the firstcommunication device according to this embodiment. The transmission unitof the second communication device 109 shown in FIG. 1 basically has thesame arrangement as that of the transmission unit of the firstcommunication device 106. Therefore, a description of the transmissionunit of the second communication device 109 will be omitted.

A transmission unit 121 of the first communication device includes atransmission buffer 123 which receives communication data 122 sent fromthe first router 104 shown in FIG. 1. The transmission buffer 123 is amemory with a relatively small capacity. A communication amountmonitoring circuit 124 monitors the communication amount by checking achange in the consumption of the buffer. The communication amountmonitoring circuit 124 then outputs a communication amount magnitudediscrimination signal 125 indicating whether the amount of communicationdata 122 sent from the first router 104 is relatively large or small.The communication amount magnitude discrimination signal 125 is input toa read clock selector 126 and the control terminal of a spreading codeselection switch 127.

In this case, the read clock selector 126 selects either a chip clock129 having a predetermined frequency output from a chip clock oscillator128 or a frequency division output 132 from a 1/N frequency divider 131which frequency-divides the chip clock 129 to 1/N, and supplies it as aread clock 133 to the transmission buffer 123. In this case, thenumerical value N is an integer equal to or more than two. The spreadingcode selection switch 127 is a switch for turning on/off the supply of aspreading code 135 output from the spreading code generating circuit 134to one input terminal of an exclusive adder 136. When the contact of thespreading code selection switch 127 is open, a fixed value of signal “0”or signal “1” is supplied to this input terminal of the exclusive adder136.

The chip clock 129 is supplied to the spreading code generating circuit134. Communication data 137 read out from the transmission buffer 123 inaccordance with the read clock 133 is input to the other input terminalof the exclusive adder 136. The exclusive adder 136 calculates theexclusive-OR (EOR) of the input spreading code 135 and the communicationdata 137, and inputs an addition output 138 to a modulator 139. Themodulator 139 modulates this and sends out a modulated output 141 to theradio communication channel 105 shown in FIG. 1. In this case, themodulator 139 performs spread-spectrum modulation when the communicationamount is relatively small, and performs general PSK modulation when thecommunication amount is relatively large.

FIG. 3 shows the arrangement of the reception unit of the secondcommunication device according to this embodiment. The reception unit ofthe first communication device 106 shown in FIG. 1 basically has thesame arrangement as that of the reception unit of the secondcommunication device 109. Therefore, a description of the reception unitof the first communication device 106 will be omitted.

A reception unit 151 of the second communication device includes ademodulator 153 which receives a reception signal 152 received from theradio communication channel 105 shown in FIG. 1 and demodulates thesignal. The demodulator 153 outputs demodulated data 154. Thedemodulated data 154 becomes one input to an exclusive adder 155 andalso becomes an input to a de-spreading synchronization circuit 156. Thedemodulated data 154 is also input to a reception clock reproducingcircuit 158 to reproduce a reception clock 159. Note that the receptionclock reproducing circuit 158 may generate a reception clock 159 havinga uniquely determined frequency without receiving the demodulated data154. The reception clock 159 is input to a de-spreading code generatingcircuit 161, one input terminal of a write clock selector 162, and a 1/Nfrequency divider 164 which inputs a frequency division output 163 tothe other input terminal of the write clock selector 162. Thede-spreading code generating circuit 161 generates a de-spreading code165 on the basis of the reception clock 159, and supplies it to ade-spreading code selection switch 166.

The de-spreading synchronization circuit 156 checks whether or notde-spreading operation can be normally performed on the basis of thedemodulated data 154. If de-spreading operation can be normallyperformed, it is discriminated that the demodulated data 154 is based onspread-spectrum communication, i.e., communication in a case wherein thecommunication amount is relatively small. Otherwise, it is discriminatedthat the demodulated data is based on communication using the generalPSK modulation scheme, i.e., communication in a case wherein thecommunication amount is relatively large. The discrimination result isthen output as a communication amount magnitude discrimination signal167. The communication amount magnitude discrimination signal 167 issupplied to the write clock selector 162 and de-spreading code selectionswitch 166. If de-spreading operation can be normally done, thede-spreading synchronization circuit 156 outputs a synchronizationtiming signal 168 and supplies it to the 1/N frequency divider 164.

When, therefore, the communication amount is relatively small and spreadspectrum is to be performed, the contact of the de-spreading codeselection switch 166 is closed by the communication amount magnitudediscrimination signal 167, and the de-spreading code 165 is input to theexclusive adder 155. The exclusive adder 155 de-spreads the demodulateddata 154 by using the de-spreading code 165, and supplies communicationdata 169 to a reception buffer 171. In this case wherein spread spectrumis performed, the synchronization timing signal 168 is supplied to the1/N frequency divider 164, and the frequency division output 163obtained by frequency-dividing the reception clock 159 to 1/N issupplied to the write clock selector 162, together with the receptionclock 159. The write clock selector 162 has received the communicationamount magnitude discrimination signal 167 indicating that thecommunication amount is relatively small. The write clock selector 162therefore selects the frequency division output 163 obtained byfrequency-dividing the reception clock 159 to 1/N and supplies it as awrite clock 172 to the reception buffer 171. When the communicationamount is relatively small and spread spectrum is performed, therefore,the communication data 169 is sampled by the frequency division output163 obtained by frequency-dividing the reception clock 159 to 1/N andwritten in the reception buffer 171.

In contrast to this, when the communication amount is relatively largeand communication is to be performed by the general PSK modulationscheme, the contact of the de-spreading code selection switch 166 isopened by the communication amount magnitude discrimination signal 167.The demodulated data 154 output from the demodulator 153 thereforepasses through the exclusive adder 155 and is supplied to the receptionbuffer 171. At this time, the write clock selector 162 selects thereception clock 159 and supplies it as the write clock 172 to thereception buffer 171. When the communication amount is large, thecommunication data 169 is sampled by the write clock 172 with a highfrequency and written in the reception buffer 171. The communicationdata 169 stored in the reception buffer 171 is sequentially read out ascommunication data 173.

FIGS. 4A and 4B show comparison between spectra when communicationschemes are switched in accordance with the communication amount in thisembodiment. FIG. 4A shows a case wherein the communication amount isrelatively small. FIG. 4B shows a case wherein the communication amountis relatively large. In this embodiment, when the communication amountis relatively small, the spreading code 135 output from the spreadingcode generating circuit 134 shown in FIG. 2 is exclusively added to thecommunication data 137 to perform spread spectrum. The chip clock 129 atthis time is N times the communication data 137 read out from thetransmission buffer 123 in accordance with the read clock 133.Therefore, the data to be transmitted is spread to increase the occupiedbandwidth, and is then transmitted.

FIGS. 5A and 5B show comparison between spectra when communicationschemes are switched in accordance with the communication amount in theprior for the sake of comparison with this embodiment. FIG. 5A shows acase wherein the communication amount is relatively small. FIG. 5B showsa case wherein the communication amount is relatively large. In theprior art, since the general communication scheme is used even when thecommunication amount is relatively small, the occupied bandwidth issmall in accordance with the data amount.

Assume a case wherein 16 kbps (Kbits per second) information exists as acase wherein the communication amount is relatively small. When thisinformation is modulated by the QPSK (Quadrature Phase Shift Keying,Quadri-Phase Shift keying) scheme and transmitted, the transmissionsymbol rate becomes 16 kHz. In this case, the QPSK scheme is amodulation scheme of making the bit information (00, 01, 10, 11) of abaseband correspond to the phase (0°, 90°, 180°, 270°) of a carrierwave. FIG. 5A shows this.

Assume that spread spectrum is performed as in this embodiment. When thespreading ratio is “128”, the product of this ratio and the transmissionsymbol rate becomes the frequency of the chip clock 129, which is 2,048kHz. In the case of spread spectrum, as shown in FIG. 4A correspondingto FIG. 5A, the necessary bandwidth increases by 128 times. However, thetransmission power required to maintain transmission quality, i.e., thetransmission power required to obtain Eb/No (the power density to noisepower density ratio per bit in a digital modulated signal) required onthe receiving side, is reduced to {fraction (1/128)}, as compared withthe conventional case wherein 2,048 kbps information is transmitted atthe same rate with an error correction rate of ½ by QPSK, because energyis spread. This corresponds to a power difference of about 21 dB.

A case wherein the communication amount is relatively large will bedescribed next. In this case, according to this embodiment, general PSKmodulation is performed instead of the spread spectrum technique. Thetransmission symbol rate is set to be the same as the rate of the chipclock 129. This is because when the communication amount is relativelylarge, the frequency of the read clock 133 for reading out thecommunication data 137 from the transmission buffer 123 shown in FIG. 2is inevitably high. In addition, in this embodiment, the centerfrequency of a carrier wave is set be the same as that obtained byperforming spread spectrum as shown in FIG. 4B. That is, when thecommunication amount is relatively small, the information rate is 16kbps, whereas the information rate is increased to 2,048 kbps to copewith a case wherein the communication amount is relatively large. Inthis case, assuming that the same transmission quality as that in thecase of 16 kbps is to be maintained, the transmission output power mustbe increased by 21 dB. Note that FIG. 5B shows a case wherein thecommunication amount is relatively large in the prior art on theassumption that communication is performed by the same communicationscheme as that used when the communication amount is relatively large inthis embodiment.

As described above, in this embodiment, when the magnitude of thetransmission rate is changed in accordance with the communicationamount, i.e., the magnitude of the traffic, since the transmissionsymbol rate itself remains unchanged, the communication interruptiontime required for the demodulator to perform resynchronization for achanged transmission rate at the time of switching transmission ratescan be eliminated. This can eliminate the necessity of a buffer area fortemporarily storing data associated with transmission/reception which isrequired to cope with a communication interruption. In this embodiment,therefore, only a minimum buffer memory for the discrimination of acommunication amount is required, allowing a great reduction in buffercapacity. This makes it possible to simplify the device and reduce thecost of the device.

Second Embodiment

In the first embodiment described above, when the amount ofcommunication data stored in the transmission buffer 123, whichtemporarily stores the communication data 122, becomes large, thecommunication amount monitoring circuit 124 connected to thetransmission buffer 123 determines that the communication amount isrelatively large, and general PSK modulation is performed. Otherwise,when the communication amount monitoring circuit 124 determines that thecommunication amount is relatively small, communication is performed bythe spread spectrum technique. Assume that the communication amountmonitoring circuit 124 discriminates, on the basis of the samethreshold, whether or not the communication amount is large. In thiscase, if the communication amount is an almost intermediate value ofthese values, information rates may be frequently switched. In order toprevent such frequency switching, a predetermined difference may be setbetween a threshold by which the information rate is switched from thehigh rate to the low rate and a threshold by which the information rateis switched from the low rate to the high rate. More specifically, thethreshold by which the information rate is switched from the high rateto the low rate is set to be smaller than the threshold by which theinformation rate is switched from the low rate to the high rate.

Third Embodiment

FIG. 6 shows an example of switching control, as the third embodiment,which is performed when communication methods are switched in a cellphone in accordance with the amount of communication data. In thisembodiment, a CPU (Central Processing Unit) (not shown) is incorporatedin the cell phone (not shown), and the CPU executes a predeterminedcontrol program to switch the communication methods. The CPU determineswhether or not the general voice communication mode is set (step S201).If the general voice communication mode is set (Y), since the amount ofcommunication data is smaller than that in another mode (to be describednext), transmission is performed by using the spread spectrum technique(step S202).

In contrast to this, if the videophone mode is set (step S203: Y), orthe Web access mode of, for example, downloading contents uponconnection to the Internet (step S204: Y), the CPU determines that theamount of communication data is relatively large, and performstransmission by general PSK modulation (step S205). As in the firstembodiment, the transmission symbol rate in spread spectrum in step S202is equal to that in PSK modulation in step S205. This can thereforeeliminate the communication interruption time required for thedemodulator to perform resynchronization for a changed transmission rateat the time of switching information rates. In addition, in the thirdembodiment, since whether the communication amount is large or small isdetermined without using the transmission buffer 123, unlike the firstembodiment, the capacity of the buffer can be further reduced.

Fourth Embodiment

FIG. 7 shows the arrangement of the transmission unit of a firstcommunication device in the fourth embodiment of the present invention,and corresponds to FIG. 2 in the first embodiment. The same referencenumerals as in FIG. 2 denote the same parts in FIG. 7, and a descriptionthereof will be omitted as needed.

In a transmission unit 121A of the first communication device accordingto the fourth embodiment, communication data 137 output from atransmission buffer 123 is directly input to a baseband modulator 139A,which in turn performs baseband modulation. Communication data 301 aftermodulation is input to one input terminal of an exclusive adder 136. Theexclusive adder 136 calculates the exclusive-OR (EOR) of a spreadingcode 135 input to the other input terminal and the communication data301, and inputs the addition output as a modulated output 141 to theradio communication channel 105 shown in FIG. 1. Other points are thesame as those in the first embodiment.

In the above embodiment, when the amount of communication data isrelatively large, general PSK modulation is performed. Obviously,however, if the clock frequency is the same as the frequency of a chipclock used for spread spectrum, information may be modulated by anothermodulation scheme.

As has been described above, according to the above embodiment, when itis discriminated that the amount of information to be transmitted perunit time is relatively small, spread spectrum is performed to increasethe bandwidth to allow transmission with the same bandwidth and the samecenter frequency as in the case wherein the amount of information to betransmitted is large. For this reason, in spite of the fact that thetransmitting side selects one of the two types of signal forms inaccordance with the amount of information and transmits the information,the receiving side need not perform resynchronization of the demodulatorwhen the signal forms are switched, because the transmission rate is notchanged. This prevents any communication interruption and eliminates thenecessity of a buffer area for handling such a problem. In addition,this eliminates the necessity of other circuits to cope with acommunication interruption, and hence can achieve simplification of thecircuit of the transmission/reception device and a reduction in cost.Furthermore, when the amount of information to be transmitted per unittime is relatively small, since the bandwidth is increased as comparedwith the prior art, stable communication can be maintained.

In addition, since the information amount magnitude discrimination meansuses two different thresholds, even if the information amount takes anintermediate value, stable circuit operation can be ensured.

Furthermore, the magnitude of the information amount may bediscriminated in accordance with the path through which information tobe transmitted is acquired or the mode used by the device instead ofmaking the information amount magnitude discrimination meansdiscriminate the magnitude of the information amount from the amount ofinformation actually processed. For example, when a homepage is browsed,relatively large volume data is downloaded. When such a path isselected, therefore, communication processing is performed upondetermination that the information amount is relatively large. When thedevice shifts to the videophone mode of performing videophone operation,relatively large volume image data is communicated. When, therefore,such a path is selected, communication processing is performed upondetermination that the information amount is relatively large. Thisallows the execution of standardized processing, and hence saves thecircuit the burden of determination. In addition, a reduction in thecapacity of the buffer memory can be achieved.

1. A variable communication system characterized by comprising: atransmission device including information amount magnitudediscrimination means for discriminating whether an amount of informationto be transmitted per unit time is relatively large or small, firstcommunication data sending means for, when said information amountmagnitude discrimination means discriminates that the amount ofinformation to be transmitted is relatively large, digitally modulatingfirst information as the information into information in a signal formhaving a predetermined bandwidth with a predetermined center frequency,and sending out the information as communication data, and secondcommunication data sending means for, when said information amountmagnitude discrimination means discriminates that the amount ofinformation to be transmitted is relatively small, digitally modulatingsecond information as the information upon performing spread spectrum toobtain the same bandwidth as the predetermined bandwidth with the centerfrequency, and sending out the information as the communication data,and a reception device including demodulation means for demodulating thecommunication data sent from said transmission device, de-spreadingappropriateness discrimination means for checking whether or not asignal after demodulation can be normally de-spread, first informationreproduction means for, when said de-spreading appropriatenessdiscrimination means discriminates that de-spreading cannot be normallyperformed, reproducing the first information from the signal afterdemodulation by said demodulation means, de-spreading means for, whensaid de-spreading appropriateness discrimination means discriminatesthat de-spreading can be normally performed, de-spreading the signalafter demodulation by said demodulation means, and second informationreproduction means for reproducing the second information from thesignal after de-spreading by said de-spreading means.
 2. A variablecommunication system according to claim 1, characterized in that saidtransmission device further comprises a transmission buffer whichsequentially receives information to be transmitted and outputs theinformation in synchronism with a predetermined read clock, and saidinformation amount magnitude discrimination means discriminates from anamount of information left in said transmission buffer whether theamount of information per unit time is relatively large or small.
 3. Avariable communication system according to claim 2, characterized inthat said first and second communication data sending means include chipclock generating means for outputting a chip clock having apredetermined frequency, transmitting-side frequency dividing means forfrequency-dividing the chip clock output from said chip clock generatingmeans at a predetermined frequency division ratio, read clock selectionmeans for, when said information amount magnitude discrimination meansdiscriminates that information to be transmitted is relatively large,setting the chip clock as the read clock, and when said informationamount magnitude discrimination means discriminates that information tobe transmitted is relatively small, setting, as the read clock, a clockobtained by frequency-dividing the chip clock by said transmitting-sidefrequency dividing means, spreading code generating means for receivingthe chip clock and generating a spreading code, transmitting-side switchmeans which receives an output from said spreading code generating meansand is turned on only when said information amount magnitudediscrimination means discriminates that information to be transmitted isrelatively small, transmitting-side exclusive addition means forcalculating exclusive-OR between information output from saidtransmission buffer in synchronism with the read clock and an outputfrom said transmitting-side switch means, and modulation means fordigitally modulating an output from said transmitting-side exclusiveaddition means and transmitting the output as the communication data. 4.A variable communication system according to claim 2, characterized inthat said first and second communication data sending means include chipclock generating means for outputting a chip clock having apredetermined frequency, transmitting-side frequency dividing means forfrequency-dividing the chip clock output from said chip clock generatingmeans at a predetermined frequency division ratio, read clock selectionmeans for, when said information amount magnitude discrimination meansdiscriminates that information to be transmitted is relatively large,setting the chip clock as the read clock, and when said informationamount magnitude discrimination means discriminates that information tobe transmitted is relatively small, setting, as the read clock, a clockobtained by frequency-dividing the chip clock by said transmitting-sidefrequency dividing means, modulation means for digitally modulatinginformation output from said transmission buffer in synchronism with aread clock, spreading code generating means for receiving the chip clockand generating a spreading code, transmitting-side switch means whichreceives an output from said spreading code generating means and isturned on only when said information amount magnitude discriminationmeans discriminates that information to be transmitted is relativelysmall, transmitting-side exclusive addition means for calculatingexclusive-OR between an output from said modulation means and an outputfrom said transmitting-side switch means and transmitting the data asthe communication data.
 5. A variable communication system according toclaim 3, characterized in that said de-spreading means and said firstand second information reproduction means include reception clockgenerating means for outputting a reception clock identical to the chipclock, de-spreading code generating means for generating a de-spreadingcode on the basis of the reception clock output from said receptionclock generating means, receiving-side switch means which receives anoutput from said de-spreading code generating means and is turned ononly when said de-spreading appropriateness discrimination meansdiscriminates that de-spreading can be performed, receiving-sideexclusive addition means for calculating exclusive-OR between an outputfrom said receiving-side switch means and a signal after demodulation bysaid demodulation means, receiving-side frequency dividing means forfrequency-dividing the reception clock at the predetermined frequencydivision ratio, write clock selection means for, when said de-spreadingappropriateness discrimination means discriminates that de-spreadingcannot be performed, selecting the reception clock, and whende-spreading appropriateness discrimination means discriminates thatde-spreading can be performed, selecting and outputting a clock obtainedby frequency-dividing the reception clock by using said receiving-sidefrequency dividing means, and a reception buffer in which an output fromsaid receiving-side exclusive addition means is written as an input inaccordance with the write clock selected by said write clock selectionmeans, and data stored in said reception buffer is set as theinformation to be transmitted.
 6. A variable communication systemaccording to claim 5, characterized in that said reception clockgenerating means comprises reception clock reproduction means forreproducing a reception clock from communication data input to saiddemodulation means.
 7. A variable communication system according toclaim 2, characterized in that said information amount magnitudediscrimination means sets a predetermined difference between a thresholdby which it is discriminated that an information amount is relativelylarge and a threshold by which it is discriminated that an informationamount is relatively small.
 8. A variable communication system accordingto claim 7, characterized in that said information amount magnitudediscrimination means sets a threshold by which it is discriminated thatan information amount is relatively large to a value larger than athreshold by which it is discriminated that an information amount isrelatively small.
 9. A variable communication system according to claim1, characterized in that said information amount magnitudediscrimination means discriminates, depending on whether or not a paththrough which information to be transmitted is acquired is apre-specified path, whether the amount of information to be transmittedper unit time is relatively large or small.
 10. A variable communicationsystem according to claim 1, characterized in that said informationamount magnitude discrimination means discriminates, depending onwhether or not a device which processes information to be transmitted isset in a pre-specified mode, whether the amount of information to betransmitted per unit time is relatively large or small.
 11. A variablecommunication system according to claim 1, characterized in that saidtransmission device and said reception device comprise a radio device.12. A variable communication system according to claim 1, characterizedin that said transmission device outputs transmission power inproportion to a transmission rate.